1. Field of the Invention
The present invention relates to an improved alumina ceramic material which is highly resistant to etching by fluorine plasmas and is characterized by reduced particle emission. The present invention further relates to a method for making the improved ceramic material, and to articles of manufacture comprising the improved ceramic material.
2. Description of the Related Art
Polycrystalline alumina ceramic materials typically are produced by the following sintering process. Powdered alumina, having a desired spectrum of grain sizes (typically having an average size of about 1 .mu.m to 3 .mu.m) is combined with a binder, and the combined alumina powder and binder are then compacted to form a green body. Commonly the green body has a composition including about 99.5 wt % alumina and about 0.5 wt % of a mixture of silica, MgO and CaO as a binder. The green body is subsequently sintered, typically in air under ambient pressure ("pressureless sintering") and at a temperature of about 1650.degree. C., for a time of about 4 hours.
During the sintering process, grain growth occurs. For example, common sintering processes yield a distribution of sintered grain sizes ranging from about 1 to about 30 .mu.m with a mean grain size of about 6 .mu.m, as determined by well-known methods such as those set forth in American Society for Testing and Materials (ASTM) standards E 1181-87 (determination of duplex grain sizes) and E 112-88 (determination of average grain sizes (AGS)). Growth of the alumina grains causes displacement of the binder phase. The displaced binder phase migrates to areas in which smaller grains are present and surround the smaller grains. Since alumina is very immiscible in silica, the primary component of the typical binder phase (see J. W. Welch, Nature, vol. 186, p. 546 et seq. (1960)), the surrounding binder phase prevents further growth of the isolated smaller grains. These isolated unsintered grains can range from about 0.1 to 0.5 .mu.m in diameter. Typically, about 1% of the alumina grains remain unsintered.
While known polycrystalline alumina ceramic materials have desirable properties, such as high strength and fracture toughness, they have proven insufficiently resistant to certain fluorine plasmas for applications in which exposure to such plasmas is required. Known aluminas are particularly susceptible to etching by fluorine plasmas, such as those generated in chemical vapor deposition (CVD) reactors during chamber cleaning processes. In such processes, plasma fluorine liberated from fluorocarbon and other fluorine-containing gases (for example, NF.sub.3 plasmas, CF.sub.4 :O.sub.2 plasmas and CF.sub.4 :N.sub.2 O plasmas) are used to remove dielectric film residues deposited in the chambers of the reactors.
Alumina per se is highly resistant to plasma fluorine; thus, sapphire, which is pure single-crystal alumina, is one of the slowest etching materials known.
Etching of polycrystalline alumina ceramic materials occurs primarily in the binder phase. As a result of etching of the binder phase, the small unsintered particles can be dislodged. The dislodged particles can then be emitted from the surface of the ceramic materials. Such polycrystalline alumina ceramic materials constitute a source of contamination when used in CVD reactors and other environments exposed to plasma fluorine.
Solutions to the particle emission problem which have been considered include the production of a polycrystalline alumina ceramic having an increased proportion of alumina, e.g., 99.9 wt % alumina and 0.1 wt % binder; use of a different binder which is less sensitive to fluorine plasma; use of a ceramic material other than alumina; and modification of the initial distribution of alumina grain sizes in the green body.
A particular application in which particle emission is problematic is in the processing of semiconductor wafers in chemical vapor deposition (CVD) systems, for example, in the "5000" apparatus provided by Applied Materials, Inc. as described by Chang et al. in U.S. application Ser. No. 08/136,529. An exemplary prior art CVD reactor is illustrated in FIGS. 1 and 2. In FIG. 1, a CVD system 10 comprises deposition chamber 12, vacuum channel 13, vacuum exhaust system 14, gas inlet means 16, gas distribution shield 17, blocker 18, wafer lift 20, baffle plate 22, lift fingers 24 and susceptor lift 26. A substrate 28, such as a semiconductor wafer, is disposed on a susceptor 30. Heating means 32, for example an external array of 1000 watt lamps directing collimated light through quartz window 36, maintains a uniform processing temperature. The deposition or reaction zone 34 lies above the substrate.
Gas distribution shield 17 is a flat annular element which surrounds blocker 18 and is removably affixed to chamber lid 38 by a plurality of aluminum clips 40, as shown in FIG. 2. Gas distribution shield 17 is typically comprises of a polycrystalline alumina ceramic material.
In a typical deposition process carried out in the illustrated CVD system, process gases (i.e., reaction and carrier gases) enter into the deposition chamber 12 via gas inlet means 16 and "showerhead" type blocker 18. The blocker 18 has numerous openings over an area corresponding to that of the substrate 28 beneath it. The spacing between the blocker 18 and the substrate 28 can be adjusted to from about 200-1000 mils (5-25 mm) to define the reaction zone 34. The blocker 18 feeds the combined process gases to the reaction zone 34. The deposition reaction is carried out, and the gases are purged from chamber 12. After each wafer is processed, the chamber is cleaned using a cleaning gas such as NF.sub.3 or a C.sub.2 F.sub.6 /NF.sub.3 /O.sub.2 gas mixture.
When gas distribution shield 17 is comprised of a polycrystalline alumina ceramic material, however, the shield is subject to etching by the cleaning gas or gas mixture as described above, with resultant particle emission. Particles having sizes from about 0.2 to 0.5 .mu.m can be emitted and can contaminate silicon wafers processed in the CVD apparatus. Particle counts of up to 200/cm.sup.2 or higher can be observed on the surfaces of the silicon wafers after 100 wafers have been processed. Such particle counts are unacceptably high.
A continuing need exists for improved polycrystalline alumina ceramic materials and methods for producing them. The materials should show high resistance to plasma fluorine, and in particular show reduced particle emission. A specific need exists for gas distribution shields, for use in a CVD apparatus, which are comprised of such an improved material.